The development of sensors that are portable and inexpensive for quantification can realize the on-site and point-of-care applications of current sensing techniques in detecting substances of significant impact on human health and environment.1-7 It can also further lead to household and personal sensors for analytes related to everyday life and health. One such successful example is glucose meter, which has been commercialized as a routine sensor for blood glucose over nearly 30 years and proven as a key element for monitoring diabetes mellitus or hypoglycemia.8,9 The personal glucose meter (PGM) is well known for its advantages of wide availability to the public, portable pocket size, low cost, reliable quantitative results, and simple operation. Thus, it has found its widespread applications in personal healthcare and provided a large and growing market. PGM can be easily and cheaply obtained from commercial sources, and has already been integrated into mobile phones such as iPhone and LG models.
Despite of PGM's success, it is still a great challenge to develop sensor systems that can detect various analytes other than glucose but also exhibit advantages of glucose meter: wide availability, portability, low cost, and quantitative analysis. In recent years, sensors that can quantitatively detect various analytes with high selectivity and sensitivity have been developed using spectroscopy,3-6 electrochemistry,1,2 magnetic resonance,7 and other analytical techniques. While some of these sensing techniques use simple instrumentation, most still need a laboratory-developed portable device, which is not widely and commercially available to the public. There are also colorimetric sensors developed for simple and on-site detection of various analytes by visible color change10-13 thus no instrumentation is required. However, these sensors are qualitative or semi-quantitative, and thus cannot provide quantitative results.